A series of new cryo-EM structures reveals a surprising twist in how the RAG complex initiates V(D)J recombination. The initial complex with substrate DNA adopts two conformations: in one, the DNA is relatively undistorted but the scissile phosphate is far from the active site, and in the other the DNA is partially melted and unwound by half a turn, which allows the scissile phosphate to dock into the active site. Similar pre-catalysis DNA melting may occur with other DDE recombinases, for which equivalent complexes with uncleaved substrate DNA are not yet available.
